1. Field of the Invention
The present invention relates to a method and a system for controlling transmission power and more particularly to a method and a system for controlling transmission power such that an SIR (Signal-to-Interference Ratio) of a received signal converges to a desired SIR.
2. Description of the Background Art
It is a common practice with a mobile communications system of the type using, e.g., a CDMA (Code Division Multiple Access) scheme to cause a base station to control, based on the SIR of a received signal, the transmission power of the individual mobile station so as to reduce interference, which is ascribable to the varying distance between the base station and the mobile station. Such a system configuration is taught in, e.g., Japanese patent laid-open publication Nos. 2001-7763 and 2002-246958 by way of example.
FIG. 3 shows a specific configuration of a conventional transmission power control system using SIR estimation. As shown, the system includes a transmission-power controlled station 3 and a transmission-power control station 4, which comprise a radio transmitter-receiver each. The transmission-power controlled station (simply controlled station hereinafter) 3 receives power control information from the transmission-power control station (simply control station hereinafter) 4 with a receiver 62 and then controls transmission power to be output from a transmitter 60 in accordance with the above information.
The control station 4 receives a signal from the controlled station 3 with a receiver 64. The control station 4 then estimates the SIR of the received signal with an SIR estimator 66, produces a difference between the SIR estimated and a target SIR, generates power control information in accordance with the difference with a control information converter 70, and then sends the power control information to the controlled station 3 via a transmitter 72. Consequently, the transmission power of the controlled station 3 is controlled such that the estimated SIR determined by the SIR estimator 66 of the control station 4 converges to the target SIR.
FIG. 4 shows a specific configuration of the SIR estimator 66. As shown, the SIR estimator 66 includes a square calculator 200 that calculates the powers of received signals 212 by squaring the individual signals 212. A mean calculator 202 produces a mean value 214 from the powers output from the square calculator 200. The mean value 214 is the mean power of the received signals 212 consisting of signal wave components and interference wave components.
On the other hand, a mean calculator 204 calculates the common-mode mean value of the received signals 212. A square calculator 206 squares the common-mode mean value output from the mean calculator 204 to thereby produce a power 216, which is the mean power S of the signal wave components of the received signals 212. A subtractor 208 subtracts the mean power 216 from the mean power 214 for thereby producing a result of calculation or difference 218. The result of calculation 218 is representative of the mean power I of the interference components. A divider (S/I) 210 divides the mean power 216 by the mean power 218 to thereby output an estimated SIR 220.
The problem with the SIR estimator 66 is that it cannot accurately estimate the SIR of the received signals 212 unless it separates only the signal wave from the signals 212. To separate the signal wave from the signals 212, it is desirable that the number of received signals 212 to be subject to common-mode mean processing be as many as possible. However, an increase in the number of received signals 212 directly translates into an increase in a period of time necessary for calculating the common-mode mean value, i.e., a mean processing time.
On the other hand, in the transmission power control system shown in FIG. 3, feedback power control is executed on the basis of the estimated SIR output from the SIR estimator 66, so that a feedback response time is determined by the mean processing time of the estimated SIR; the longer the mean processing time, the greater the delay of the feedback response time. Because the delay of the feedback response time is apt to cause transmission power sent from the controlled station 3 to oscillate, the mean processing time of the estimated SIR must be reduced to reduce the oscillation of transmission power. This, however, makes it impossible to separate only the signal wave from the received signals 212. As a result, the estimated SIR output from the SIR estimator 66 differs from the actual SIR and prevents the SIR of the received signals 212 from accurately converging to the target SIR despite the transmission power control.